1. Field of the Invention
The present invention relates to an electronically controlled fuel injection system or a rotational speed control device in a diesel engine mounted in a motor vehicle.
Various kinds of auxiliary devices have recently come to be mounted in motor vehicles in order to meet variety of demands. Of these devices, some are designed to be driven by rotation of an engine; and there are many large loads which change by their operation the rotational speed, particularly an idle speed, of the engine.
For example, an air-conditioning system, a power steering system, and a device, such as a defogger, which consumes much electric current, have such a shortcoming that as it operates, the load torque of a generator (alternator) to the engine increases, resulting in a large drop in the rotational speed and possibly the engine stopping. An example of a conventional engine, especially a gasoline engine, will be described with reference to drawings.
FIG. 5 is a block diagram of a conventional engine speed control system. In this drawing, numeral 1 denotes a voltage setting circuit which outputs a set signal having a voltage level in accordance with a desired target speed. Both the set signal and a detection signal, having a voltage level corresponding to an actual engine speed outputted from the speed detection circuit 5, are supplied to a subtractor 11. The subtractor 11 calculates a difference between the set signal and the detection signal, and outputs it to a controller 2. This controller 2 often includes a proportional integral controller having a circuit for amplifying a deviation signal and a circuit for integrating this deviation signal, both of which are in parallel connection.
An actuator 3 is designed to regulate the ignition timing or the intake air flow rate of an engine 4 in accordance with the voltage output of the controller 2.
The speed control system ranging from the input end of the actuator 3 to the engine 4 and the output end of the speed detection circuit 5 shown in FIG. 5 may be expressed in terms (345) of one transmission function as shown in FIG. 6.
Next, operation of a conventional engine speed control system will be explained by referring to FIG. 5. First, suppose that a target voltage signal corresponding to a target speed (generally, this target speed varies with the operating point of engine, 800 to 900 rpm when the air-conditioning system is on at the idle speed of engine) is outputted from the setting circuit. Next, the subtractor 11 calculates a difference between this target voltage signal and the voltage signal corresponding to an actual engine speed outputted from the speed detection circuit 5, producing a deviation signal. Then, this deviation signal is amplified proportionally and integrally by the proportional integral controller 2, which sends this voltage signal as a manipulated variable to the actuator 3.
The actuator 3 controls the ignition timing or the intake air flow rate of the engine 4 in accordance with this voltage signal. The engine 4 operates at an actual speed corresponding to the ignition timing or the intake air flow rate commanded by the actuator 3. The speed detection circuit 5 generates a voltage signal corresponding to this actual number of revolutions. The voltage signal thus generated in accordance with this actual speed is fed back to the subtractor 11 side.
By the way, it goes without saying that such a feedback control system, in a steady state, settles down where the deviation signal becomes zero. At this time, the voltage signal corresponding to the target speed and the voltage signal corresponding to the actual speed become equal, and accordingly the engine speed equals the target speed. That is, in the steady state, the engine speed is so controlled as to be always equal to the target speed.
Next, the operation of the speed control device in a transient state will be explained, using a typical example of a transient state wherein a load (for example the air-conditioning system) is suddenly applied at an idle speed of engine.
Now, suppose that when the control system shown in FIG. 5 is in a steady state, a load is abruptly applied to the engine, resulting in a sudden drop of engine speed. At this time, since the voltage signal outputted from the speed detection circuit 5 also drops, the deviation signal becomes a positive voltage signal, whereby operating the control system to raise the speed of the engine 4 through the proportional-integral controller 2 and the actuator 3, thus the engine recovers to the original target speed.
In order to increase the engine speed to the original target speed as quickly as possible within this process, it is evidently desirable to increase proportional and integral gains at the proportional-integral controller 2 which receives the deviation signal and to give the actuator 3 a great voltage signal in relation to the same deviation signal. That is , it is possible to quickly raise the lowered engine speed back to the target speed by increasing the sensitivity of the control system.
Generally, it is very important to increase the sensitivity of the control system by increasing the proportional and integral gains of the proportional-integral controller in the feedback control system as stated above in order to (A) quickly eliminate the influence of disturbance and (B) gain a specific result of control irrespective of characteristic variation or dispersion of an object of control. In an actual engine speed control system, however, it is commonly a matter of great difficulty to increase the sensitivity of the control system because increasing the sensitivity of the control system causes engine hunting to occur. Commonly, in the case of the engine, when, for example, the actuator 3 operates to control the intake air flow rate, the transmission characteristic from the intake air flow rate to the engine speed shows the following shortcomings: (A) the presence of a secondary delay factor by which the phase delays 180 degrees, and the occurrence of a tertiary delay which causes a phase delay of 270 degrees when an actuator delay is included, and (B) the presence of an idle time factor resulting from a stroke delay; and therefore if the sensitivity of the control system is increased (to a high gain), the control system itself becomes unstable, causing hunting to occur. This occurrence of hunting caused by the increase in the proportional and integral gains is empirically well known. It is, therefore, necessary to theoretically prove it as a common phenomenon.
This point will be described in detail by referring to FIG. 6 and using equations. In FIG. 6, let Gc(S) and G.sub.345 (S) e.sup.-SL be respectively the functions of the proportional-integral controller 2 and the transmission function (345), r be the voltage signal of the setting circuit 1, and y be the output (voltage signal) of the transmission function (345), and the closed-loop transmission function y/r will be given by the following equation. ##EQU1## Therefore, a characteristic equation which governs the stability of the control system will be given by the following equation: EQU 1+Gc(S)G.sub.345 (S)e.sup.-SL =0 (3)
where Gc (S) is the transmission function of the proportional-integral controller 2.
As is well known, stability analysis using the equation (3) can be executed by drawing a Nyquist diagram. The stability of the control system will be analyzed by actually drawing a Nyquist diagram.
First, let K be a proportional gain and Ti be an integral gain (integral action time), and Gc (S) which is proportional-integral is given by ##EQU2## In the meantime, the transmission function G.sub.345 (S) from the actuator to the engine can be accurately approximated with the secondary delay of ##EQU3## when the actuator makes a very quick response. Here, T is a time constant, and depends upon the engine speed, the moment of inertia of a flywheel, and the capacity of a surge tank. The time constant is of the order of 0.3 sec at a balanced engine speed No=750 rpm. When the delay time L is equal to a time required for four strokes, 4.times.60/(2.times.No)=0.16 sec at the balanced engine speed No=750 rpm. By substituting S=j.omega. into the equations (4) and (5) to give modified equations, .omega.KTi=.omega.T.times.(KTi/T), .omega.Ti=.omega.t.times.(Ti/T), and .omega.L=.omega.T.times.(L/T),and by drawing a Nyquist diagram using K and Ti as parameters, a diagram in FIG. 7 for example is obtainable. In this drawing, a full line indicates the stability of the control system when K=0 and Tn=Ti/T=1 (namely, when only an integrator is used as a controller) (in this case, Ln=L/T=0.5). As is clear from the drawing, the phase is 180 degrees at the frequency f=0.37 Hz, and an absolute value is 0.96, from which it is understood that the control system is at the limit of stability (in actual operation, these values are negligible). From each Nyquist diagram using K and Ti as parameters, it is understood that the control system will become unstable at a frequency ranging from 0.3 Hz to 0.7 Hz. In the meantime, according to an experimental result, within this frequency range the idle speed control system becomes unstable and the hunting occurs within the frequency of 0.3 Hz to 0.7 Hz. From this, a result of the analysis described above is understood to agree very well with a result of experiments. From this analysis the range of K and Ti where the control system stability is obtainable will be K=1 to 2 and Ti/T being above 1. This result also agrees with the result of experiments. From the above-mentioned analysis, it is understood that (A) the control system will become unstable (both the proportional and integral gains can not be increased) if the proportional gain K of the idle speed control system is held under about 2 and the integral time Ti held greater than 0.3 sec, and that (B) accordingly, it is impossible to improve the sensitivity (high gain) of the control system, resulting in a poor response characteristic (follow-up characteristic) to disturbance and accordingly in an engine stop in the event of sudden application of a great load.
Another cause of the poor response characteristic (follow-up characteristic) to disturbance of the idle speed control system and the occurrence of engine stop in the event of sudden application of a great load lies in that only the intake air flow rate is controlled, without accurately grasping the dynamic characteristics of the alternator and accordingly without taking any reasonable and effective measure in relation to the load. This will be described in detail by referring to FIG. 8 and using an example particularly of an electric load disturbance.
In FIG. 8, numerals 11a to 11d denote subtractors; numeral 100 represents the primary delay characteristic of an intake manifold; numeral 101 represents characteristics in connection with a torque produced by fuel combustion in the engine; numeral 102 represents a primary delay in connection with a rotating section; numeral 103 represents a feedback gain of a regulator; numeral 104 represents a primary delay characteristic of a field circuit; numeral 105 represents a torque conversion efficiency; and numeral 106 represents a set voltage for the regulator. Above the broken line is shown the dynamic characteristic of the engine, and under the broken line is shown the dynamic characteristic of the alternator. The dynamic characteristic of the alternator is obtained by formulating variations from a balanced state, from relationships established among the field current If, load current Ia, and excitation voltage Ea. Complicated relationships will not be described in detail because it will disturb the qualitative understanding of phenomena; hereinafter, therefore, only brief description will be given with reference to a block diagram. In this block diagram, the operation of the voltage regulator mounted to the alternator is expressed by a feedback loop including the feedback gain Kf. The exciting voltage Ea is proportional to the product of alternator rotor speed (engine speed x pulley ratio) and the field current If, and the torque T demanded of the engine is proportional to the product of the load current Ia, the alternator rotor speed (engine speed.times.pulley ratio) and the field current If. Therefore, formulation of variations (expressed with .DELTA.) from values of these various quantities in a balanced state will give the dynamic characteristic of the alternator below the broken line in FIG. 8. Here, To denotes a conversion coefficient for providing a torque demanded of the engine in a balanced state. Also, the variations, excepting that of the torque, are normalized by values all in the balanced state (indicated by *).
Using the same diagram, how deeply the characteristics of the alternator is related with engine speed stability will hereinafter be described. In this diagram, suppose that the load current has increased by .DELTA.Ia* and the torque by To..DELTA.Ia*. Normally, since an increase in the intake air flow rate has an influence upon the torque after some delay, an increase in the torque affects the engine speed with delay, lowering the engine speed by .DELTA.N*. Thus this lowered engine speed reduces the exciting voltage of the alternator, and the voltage regulator functions to increase the field current by .DELTA.If*, thereby further increasing torque demanded of the engine, to To (.DELTA.Ia*+.DELTA.If*). Namely, the more the engine speed decreases, the more the alternator increases the torque demanded of the engine, further lowering the engine speed. In other words, the alternator operates towards deteriorating the stability of the engine speed. From this it is clear that the use of a conventional speed control system which controls only the flow rate of intake air without taking into account the characteristics of the alternator described above, has a low capacity to eliminate speed variations caused by load disturbance.
There have been proposed various devices for improving the above-described conditions. There is often adopted such a computerized method (a kind of feed-forward function) wherein a switch signal from an air-conditioning system for example is fed into a computer, which, upon knowing the start of operation of the air-conditioning system before the actual application of the load of the air-conditioning system to the engine, drives the actuator (3) prior to the actual application of the load to the engine. According to this method, however, if there exists a large delay between the supply of the switch signal to the computer and the actual application of load of the air-conditioning system to the engine, the engine speed in some cases shows a sudden rise and then a drop, giving a driver an unpleasant impression.
A feedback control system shown in FIG. 9 has been proposed as one example of such improvements in Japanese Examined Patent Publication No. 61-43535. In this drawing, numeral 6 denotes a detecting circuit which outputs a detection signal, or voltage, corresponding to a decrease in the engine speed. The detection signal outputted from this detecting circuit 6 and a detection signal outputted from the speed detecting circuit 5 are added by an adder 12, and a result of this addition is outputted to the subtractor 11.
Next, the operation shown in FIG. 9 will be described. In this drawing, suppose that this control system in a steady state as previously stated is suddenly affected by load disturbance, resulting in a rapid decrease in the engine speed. In this case, circuits ranging from the setting circuit 1 to the speed detecting circuit 5 function in an identical manner. In FIG. 9, however, the voltage proportional to the deceleration of the engine is excessively fed back from the detecting circuit 6 which outputs an output signal of voltage proportional to the deceleration. Thus a deviation signal will become greater as compared with the operation shown in FIG. 5 and accordingly the original target speed is recovered much more rapidly as compared with FIG. 5.
The engine can recover the original target speed more rapidly than FIG. 5 because of the implementation of this one kind of feed-forward function. To accomplish the initial object of feed-forward compensation, the engine speed must vary. However, since this variation in the engine speed delays operation, it is difficult to totally eliminate speed variation.
According to Japanese Examined Patent Publication No. 61-53544, the control of ignition timing by the actuator 3 shown in FIG. 5 has been proposed. Generally, either the intake air flow rate or the ignition timing is controlled in order to control the engine speed. In this case, the ignition timing, making a quicker response than the other, is controlled, whereby the effect of disturbance to lower the speed can be removed quickly. However, because of a limited range of speed that can be controlled by the ignition timing, the above-mentioned method is not so effective when a great load exceeding the range is applied.
As explained with reference to FIGS. 5 and 9, the conventional engine speed control device is capable of quickly eliminating the effect of load disturbance on the engine and recovering the engine speed to the original target speed; however, as only either the intake air flow rate or the ignition timing is controlled without considering the dynamic characteristics of the alternator, its effect is limited.